Fluororesins have superior nontacky properties and chemical resistance and, hence, are extensively used in the food industry, the electrical and machine industry, etc. Applications thereof include cooking utensils such as frying pans, pots, hot plates, and the inner pots of rice cooker-warmers, food vessels or containers, and electrical products.
There are the following three conventional techniques mainly employed in the above field.
(1) A technique in which the surface of an aluminum or aluminum alloy base is etched and then coated with a dispersion of polytetrafluoroethylene (PTFE), the coating is dried and baked, and a formed article, for example a pot, is obtained from the resulting coated base by press forming.
(2) A technique in which the surface of an aluminum or aluminum alloy base is etched, a tetrafluoroethyleneethylene copolymer (ETFE) film is then laminated to the etched surface, and a formed article, for example a pot, is obtained from the resulting laminated base by press forming.
For example, in the case of pots for boiling rice or containing boiled rice (hereinafter, referred to as "rice pots"), the inner surface of the pots is coated with a fluororesin such as those mentioned above because boiled rice is highly apt to adhere to the inside of the pots.
Technique (2) above is inferior to technique (1) above in that ETFE is poor in heat resistance, so that technique (1) has been far more often employed than technique (2).
(3) A technique that is substantially the same as (1) above except that a filler such as carbon is incorporated in the PTFE.
The above-described conventional techniques, however, have had the following problems.
(A) Since PTFE and ETFE are soft resins and are translucent or transparent, articles coated with these fluororesins are defective in that the coatings are prone to receive scratches and that in the case where such articles are used, for example, as rice pots, the aluminum base is discolored due to the penetration of steam, etc. during use. Because of such poor scratch resistance and the discoloration problem, these products have short lifetimes.
(B) Articles produced by the above technique (3) are defective in nontacky properties and corrosion resistance (e.g., the coatings are apt to have pinholes) and, hence, are not used as rice pots.
A further problem accompanying the conventional techniques from the standpoint of color is that the coatings of the conventional fluororesins mainly are colorless and transparent to allow the colors of the bases to be seen therethrough or are monochromatic such as white, gray, or black, and at the most, they are of metallic colors obtained by imparting metallic luster to these monochromatic colors.
In the field of consumer products, particularly those for home use, there have recently been increasing needs of a wide range of color selection, and color, which has been thought to be a second-order function, is becoming an indispensable function. This can be seen in recently marketed refrigerators which can be ordered with particular colors.
Coloring of fluororesin coatings, however, has the following problem. Since there are almost no good solvents for fluororesins due to their excellent chemical resistance, fluororesin-coated articles are obtained by a process in which fluororesins in the form of an aqueous dispersion or a powder are coated on substrates and then baked at high temperatures. In such coating processes, organic colorants, e.g., dyes, cannot be used because the colorants are denatured by heat.
Coloring with inorganic pigments such as red iron oxide and ultramarine, on the other hand, has problems that if inorganic pigments are used in small amounts, the fluororesin coatings cannot be sufficiently colored, while if the pigments are incorporated in large proportions, important properties of the fluororesin coatings, such as adhesion to the substrates and surface-nontacky properties, are greatly impaired.
For the above reason, carbon black and titanium oxide have been incorporated, which match with the colors of the base substrates to give good colors even if they are incorporated in such small amounts that the above-mentioned important properties of fluororesin coatings are not impaired greatly. Thus, monochromatic colors only, such as white, gray and black, have so far been obtained by incorporation of carbon black and titanium oxide.
On the other hand, in the case of fluororesin compositions coated on articles that are required to have wear resistance, such as frying pans, improved wear resistance of the coatings has been attained by incorporation of mica which has been practically used in rubbing materials for a long time. Such articles have metallic tones due to the secondary effect of mica. However, where mica only is incorporated as a filler, considerable unevenness of color occurs in some coating techniques if the thicknesses of the coatings are not uniform, because the opacifying power of mica as a pigment is insufficient. Therefore, mica has commonly been used in combination with a metal powder, silica, alumina, carbon, titanium oxide or the like.
JP-A-52-10386 and International Publication JP Sho-56-501277 disclose a method for attaining both wear resistance and nontacky properties, which comprises forming a base coating comprising a fluororesin and incorporated therein mica, pigment-coated mica, etc. and then coating this base coating with a fluororesin only or with a fluororesin composition comprising a fluororesin and a reduced proportion of mica. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, articles produced by the above method have been found, by the present inventor, to have the following problem. The coatings on these articles have insufficient adhesion to the substrates and, hence, where such articles are used as the inner pots of rice cooker-warmers or other rice pots, which are required to have outstanding corrosion resistance, the metal substrates corrode due to water and salts penetrated through a small number of minute defects present in the coating films, and the resulting metal oxides lift up the coating films to damage the pots.
In an attempt to improve such insufficient adhesion, JP-A-56-103257 proposes to incorporate a heat-resistant binder. However, this technique has a problem that not only satisfactory adhesion has not been obtained, but the nontacky properties are impaired by the incorporation of the heat-resistant binder.
The present inventor found that improved coated articles can be obtained by combining the above-described three techniques to produce a fluororesin-coated article by forming a base coating comprising a fluororesin and incorporated therein mica and a heat-resistant binder and then coating this base coating with a fluororesin only. Although the coated article thus obtained had been considerably improved compared to conventional ones, the adhesion of the coating to the substrate was still insufficient.
For the purpose of obtaining coated articles of a variety of colors, chromatic inorganic pigments have been tried. However, if they are used in sufficient amounts to obtain desired coloring properties, the adhesion of the resulting coatings to the substrates is weakened even more. Hence, coated articles of a variety of colors that can be put to practical use have not yet been obtained.
Furthermore, when coated substrates are subjected, in particular, to press forming to produce inner pots for rice cooker-warmers or the like, the coating films are stretched by pressing and, as a result, pinholes, peeling of the coating films, and breaking of inner part of the coating films are apt to occur. Therefore, except those obtained by coating electrochemically or chemically etched aluminum or aluminum alloy bases with a fluororesin containing no filler, none of the conventional coated articles has been able to be put to practical use.